1. Field of the Invention
Embodiments of the present invention describe systems and methods for applications using sub 200 fs laser pulses in the NIR, e.g. for non-linear microscopy or generating Terahertz (THz) radiation, using a fiber delivery with a higher order mode (LP02) fiber output without pre-chirping.
Pre-chirping in the context of this document shall refer to a method that intentionally introduces anomalous dispersion with bulk optics prior to the fiber delivery with the aim to obtain shortest possible pulse durations right after the fiber delivery output. Further bulk optical components like gratings, prisms, grisms and dispersive mirrors and so on, are referred to as any arrangements de-coupled from the fiber delivery module that are supposed to partly or fully compensate the dispersion of the involved optical fibers.
2. Description of the Related Art
Delivery of ultrashort laser pulses in the near infrared wavelength range is attracting more and more attention due to the rising number of industrial and medical applications that utilize very short optical pulses to outperform conventional measurement methods in speed, quality and resolution, and due to the fact that femtosecond light sources have been matured in their size, stability and user friendliness. However, in all cases the high flexibility of a fiber link is required, and such a fiber link is difficult to make using standard fibers commercially available and without using pre-chirping with bulk optical components. One example is generation of THz pulses. Generation of THz pulses is very efficient in the NIR wavelength range using fs laser pulses. There are commercial and practical interests in having a fiber delivery that enables transport of femtosecond pulses from the light source to a THz system. THz generation in industrial applications is typically for security purposes (detection of explosives or dangerous liquids) or for material inspection.
Fiber delivery of ultrashort pulses in the 800 nm and 1030 nm wavelength range is traditionally done using a normal dispersive fiber such as a standard-single mode fiber or LMA fiber in combination with a bulk optic dispersive element. The dispersive element has anomalous dispersion that matches the normal dispersion of the fiber and ideally provides net zero group delay dispersion. The bulk optic dispersive element introduces a chirp to the pulse prior to launching into the fiber, which compensates the chirp that will be introduced in the normal dispersive fiber; hence, the term “pre-chirping.” The bulk optic anomalous dispersive element typically comprises diffraction gratings, prisms, combination of gratings and prisms, or chirped mirrors, and contribute significantly to the cost, size, and practical use in terms of alignment and stability of the fiber delivery system.
Many different technological approaches have been reported that facilitate the transmission of laser pulses in the NIR by optical fibers. In one example, 82 fs light pulses at 800 nm were demonstrated after altogether 0.75 m of single mode fiber by use of a combination of temporal and spectral compression. In another example, by the help of a grating compressor, 140 fs pulses can travel through a 1.3 m microstructured fiber. In another example, without pre-chirping, 170 fs pulses were delivered through a hollow-core photonic crystal fiber (PCF). Using the large dispersion of a higher order mode fiber, sub 150 fs laser pulses were sent over 2 m of optical fiber in another example. In yet another example, 25 fs light pulses were obtained after 1.6 m optical fiber by using dispersive mirrors and gratings for pre-chirping. The same approach facilitates 160 fs laser pulses to pass through 45 m optical fiber with transmission efficiencies up to 40%. However, fiber delivery of laser pulses shorter than 200 fs generally has to rely on pre-chirping with space consuming and alignment sensitive bulk optics.
In addition, MultiPhoton Fluorescence Microscopy is a technique to obtain a very precise image, e.g., of biological samples, using a light source of twice the wavelength or higher multiple of the wavelength. The idea is to utilize a narrow focus and short optical pulses such that 2 or more photons will excite the sample at a very well defined location, e.g., at the very center of the focused light. Only light in this well-defined volume will have enough intensity to excite the sample with two or more photons simultaneously and generate fluorescence. If you would use light at the fundamental wavelength using only one photon to excite the sample, you would get fluorescence from all over the place and your image gets blurred.
Thus, there is a need for systems and methods to deliver sub 200 fs laser pulses in the NIR through an optical fiber without the need for pre-chirping with bulk optics.